The present invention relates to a polypropylene resin composition having excellent impact resistance.
Crystalline polypropylene has excellent rigidity, excellent heat resistance, a small coefficient of thermal expansion and good moldability and accordingly is in wide use as various molded articles However, it has relatively low impact resistance at low temperatures. In order to improve the impact resistance of crystalline polypropylene, there has been widely effected a method comprising adding to a polypropylene an ethylene-.alpha.-olefin copolymer rubber or a polyethylene.
This method, however, has a problem of reducing the rigidity, heat resistance, coil resistance, etc. of polypropylene.
Meanwhile, it has been tried to add a polyamide to a polypropylene to improve the heat resistance, oil resistance, tensile strength, etc. of polypropylene. However, since a polypropylene and a polyamide has no compatibility with each other, cleavage occurs after they are melt mixed and no desired material can be obtained. Hence, there is generally used a modified polypropylene obtained by grafting a polypropylene with an unsaturated carboxylic acid or its derivative (see Japanese Patent Publication No. 30945/1970). In this improved method, the compatibility between a polypropylene and a polyamide is improved and the polyamide is dispersed in the polypropylene matrix in microstructures. In this improved method, however, the impact resistance of polypropylene is improved hardly. Hence, there was proposed a method comprising adding to a polypropylene and a polyamide an agent for making them compatible with each other and for improving the impact resistance of the polypropylene, that is, a modified ethylene-.alpha.-olefin copolymer rubber obtained by grafting an ethylene-.alpha.-olefin copolymer rubber with an unsaturated carboxylic acid or its derivative [see Japanese Patent Application Kokai (Laid-Open) No. 149940/1984]. In this method, the compatibility between polypropylene and polyamide and the impact resistance of polypropylene are improved; however, the resulting composition is not satisfactory in dispersibility of polyamide and low temperature impact resistance.
As a measure for the compatibility and impact resistance of a polypropylene-polyamide mixture, there can be mentioned the particle diameters of finely dispersed polyamide domains. It is necessary to disperse these polyamide domains in desired particle diameters (0.3-1.0 .mu.m). In order to attain such dispersion, it is necessary not only to modify a polypropylene or an ethylene-.alpha.-olefin copolymer rubber as mentioned above but also to specify the correlation of the melt viscosities of a modified polypropylene and a polyamide when they are melt mixed at a given shear rate.